The field of the invention is magnetic resonance angiography ("MRA"), and particularly, dynamic studies of the human vasculature using contrast agents which enhance the NMR signals.
Diagnostic studies of the human vasculature have many medical applications. X-ray imaging methods such as digital subtraction angiography ("DSA") have found wide use in the visualization of the cardiovascular system, including the heart and associated blood vessels. Images showing the circulation of blood in the arteries and veins of the kidneys and the carotid arteries and veins of the neck and head have immense diagnostic utility. Unfortunately, however, these x-ray methods subject the patient to potentially harmful ionizing radiation and often require the use of an invasive catheter to inject a contrast agent into the vasculature to be imaged.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement is referred to in the art as a "view" and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. The total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
MR angiography (MRA) has been an active area of research. Two basic techniques have been proposed and evaluated. The first class, time-of-flight (TOF) techniques, consists of methods which use the motion of the blood relative to the surrounding tissue. The most common approach is to exploit the differences in signal saturation that exist between flowing blood and stationary tissue. The improvement in blood-tissue contrast is due to the stationary tissues experiencing many excitation pulses and becoming saturated. Flowing blood, which is moving through the excited section, is continually refreshed by spins experiencing fewer excitation pulses and is, therefore, less saturated. The result is the desired image contrast between the high-signal blood and the low-signal stationary tissues.
MR methods have also been developed that encode motion into the phase of the acquired signal as disclosed in U.S. Pat. No. Re. 32,701. These form the second class of MRA techniques and are known as phase contrast (PC) methods. Currently, most PC MRA techniques acquire two images, with each image having a different sensitivity to the same velocity component. Angiographic images are then obtained by forming either the phase or complex difference between the pair of velocity-encoded images. Phase contrast MRA techniques have been extended so that they are sensitive to velocity components in all three orthogonal directions.
More widespread application of either TOF or PC MRA techniques is hampered by the presence of a variety of deleterious image artifacts, which can mask and, in some cases, even mimic pathology. These artifacts generally result in a lower specificity, as well as compromised sensitivity.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 the trick is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. This is not an easy timing to achieve as part of a routine clinical procedure because the delay time between intravenous injection to arrival in the arterial vasculature of interest is highly patient-dependent. Therefore, some means is required for determining this delay time and synchronizing MR data acquisition to the contrast bolus profile. Such synchronization is necessary to provide adequate vessel contrast and to prevent artifacts such as edge enhancement of the vessel. Various means have been developed to provide accurate timing including: use of a small test injection of contrast as described by J. K. Kim, R. I. Farb, and G. A. Wright, Test Bolus Examination in the Carotid Artery at Dynamic Gadolinium-enhanced MR Angiography, Radiology, 1998, 206:283-289, real-time line scanning described by T. K. Foo, S. Manojkumar, M. R. Prince, and T. L. Chenevert, Automated Detection of Bolus Arrival and Initiation of Data Acquisition in Fast, Three-dimensional, Gadolinium-enhanced MR Angiography, Radiology 1997, 205:137-146, real-time fluoroscopic imaging as described by A. H. Wilman, S. J. Riederer, B. R. King, J. P. Debbins, P. J. Rossman, R. L. Ehman, Fluoroscopically Triggered Contrast-Enhanced Three-dimensional MR Angiography with Elliptical Centric View Order: Application to the Renal Arteries", Radiology 1997, 205:137-146.
The in vivo contrast enhancement profile provided by the passage of a contrast agent bolus closely matches a gamma-variate function as described by the general equation: EQU C(t)=Ate.sup.-.zeta.t.
As shown in FIG. 4, as a result of the contrast agent passage the acquired NMR signal is enhanced considerably for a short time interval and then the enhancement tapers off. Consequently, even if the MR acquisition is accurately synchronized to the contrast bolus, only a small number of views (usually the central k-space views) are acquired while the T1 shortening associated with high contrast agent concentration is at its peak. The bulk of the image is acquired while contrast concentration is decreasing and T1 time is increasing.
The measured signal intensity due to a series of equally spaced radio frequency (rf) pulses can be described by the following equation for the transverse magnetization that is produced: ##EQU1##
The flip angle .alpha., which maximizes the measured signal is termed the "Ernst" angle. Non-contrast enhanced gradient recalled echo (GRE) imaging uses flip angles set deliberately near the Ernst angle of the spins being imaged to increase the T1-weighted contrast. This angle is expressed quantitatively as: EQU .alpha..sub.E =cos.sup.-1 (e.sup.-TR/T1).
Typically, the flip angle is fixed at this value for acquisition of the complete NMR data set.